UCL Monday 14 Mar 2005

1. UCL Monday 14 Mar 2005 Forensic inference – is the law a ass? Ian Evett Forensic Science Service  The Forensic Science Service 2004

2. “If the law supposes that,” said Mr. Bumble... “the law is a ass – a idiot.” Charles Dickens: Oliver Twist ch 51.

3. Match Either: the defendant left the crime stain Or: some unknown person left the crime stain What is the probability that the crime profile would match the defendant if he had left it? What is the probability that the crime profile would match the defendant if some unknown person had left it?

4. What is the probability that the crime profile would match the defendant if he had left it? If the profile is not a mixture, then this probability is, effectively, one It is important to bear in mind that in more complex cases - such as paternity and mixture cases - this probability is substantially less than one and need careful consideration

5. What is the probability that the crime profile would match the defendant if some unknown person had left it? Statisticians agree that this is correctly termed a match probability

6. DNA profiling is a model for the evolution for all scientific evidence Ideally, we would like to see our opinions supported by statistics in all cases. However, it is clear that the courts are poorly equipped to ensure that the statistics are presented to the jury in a method that best informs their decision making.

7. DNA Match The probability of finding that profile if the crime stain had come from someone else is 1 in a billion But the court is interested in the probability that the crime stain was left by the defendant

8. That probability depends not just on the scientific evidence... But the court is interested in the probability that the crime stain was left by the defendant ...but also on whatever other evidence there is in the case

9. There are various ways of setting the scientific evidence in the context of a trial The probability of finding that profile if the crime stain had come from someone else is 1 in a billion The prosecutor’s fallacy Doheny and Adams judgment Probability of another person Prior/posterior illustrations

10. The prosecutor’s fallacy Match This second statement does not follow from the first... The probability of finding that profile if the crime stain had come from someone else is 1 in a billion So it follows that there is a 1 in a billion probability that the crime stain had come from someone else... Hence it is almost certain that the crime stain came from the defendant This mistake was made in the Deen, Doheny and G. Adams trials

11. R v. Doheny Match ...I calculated the chance of finding all of those bands...to be about 1 in 40 million The likelihood of it being anybody other than Alan Doheny? Is about 1 in 40 million.

12. R v. G Adams Match Is it possible that the semen could have come from a different person from the person who provided the blood sample? It is possible but it is so unlikely as to really not be credible. I can estimate the chances of this semen having come from a man other than the provider of the blood sample. I can work out the chances as being less than 1 in 27 million

13. Doheny and Adams judgment When the scientist gives evidence it is important that he should not overstep the line which separates his province from that of the jury Match He will properly explain to the Jury the nature of the match... He will...give the Jury the random occurrence ratio – the frequency with which the matching DNA characteristics are likely to be found in the population at large

14. Doheny and Adams judgment Match Note that it is difficult to imagine a case in which it is reasonable to consider every man, woman and child in the UK to be a possible perpretrator... Provided he has the necessary data...it may be appropriate for him then to say how many people with matching characteristics ...are likely to be found in the UK ...nevertheless, let us see where this leads to

15. Doheny and Adams judgment Match It is difficult to believe that statements about fractions of people are helpful to the jury The probability of finding that profile if the crime stain had come from someone else is 1 in a billion How many other people in the UK are likely to have the same profile? Approximately one twentieth of a person

16. Doheny and Adams judgment Problems with the Doheny and Adams prescription Conflicting evidence: eyewitness, alibi, geography, other scientific evidence Concept of pool of equiprobable suspects Relations of the defendant Fractional people

17. Match Doheny and Adams judgment When the scientist gives evidence it is important that he should not overstep the line which separates his province from that of the jury The scientist should not be asked his opinion on the likelihood that it was the Defendant who left the crime stain, nor when giving evidence should he use terminology which may lead the jury to believe that he is expressing such an opinion

18. Match Either: the defendant left the crime stain Or: some unknown person left the crime stain The evidence is a million times more probable if the first proposition is true This is extremely strong support for the first proposition That statement is counter to the Doheny/Adams judgment.

19. Either: the defendant wrote the questioned handwriting Or: some unknown person wrote it. In my opinion, there is extremely strong support for the first proposition Thank you, Dr Bloggs, that is very helpful.

20. In my opinion, the crime mark and control print were made by the same person. Thank you, Mr Holmes - that is amazingly helpful.

21. R v. Dennis John Adams Rape: Hemel Hempstead DNA match between profile from vaginal swab and Mr Adams Match probability 1 in 200 million Victim said that Mr Adams did not look like the rapist Mr Adams had an alibi

22. The central problem in this case was that of combining statistically based scientific evidence in support of the prosecution... R v. Dennis John Adams ...with non scientific evidence in support of the defence

23. There were two trials and two appeals. R v. Dennis John Adams To cut a long story short... ...defence argued that the problem should be approached by inviting the jury to quantify their assessment of the non- scientific evidence ...then combining all of the various elements by means of Bayes’ theorem

24. R v. Dennis John Adams It seems to us that the difficulties which arise in the present case stem from the fact that, at trial, the defence were permitted to lead before the jury evidence of the Bayes theorem. Judgment: First appeal But we have very grave doubt as to whether that evidence was properly admissible, because it trespasses on an area peculiarly and exclusively within the province of the jury, namely the way in which they evaluate the relationship between one piece of evidence and another.

25. R v. Dennis John Adams ...the attempt to determine guilt or innocence on the basis of a mathematical formula, applied to each separate piece of evidence, is simply inappropriate to the jury’s task. Judgment: First appeal Jurors evaluate evidence... by the joint application of their individual common sense and knowledge of the world...

26. R. v. D.J. Adams Retrial Defence argued that the non-DNA evidence should be evaluated numerically. All evidence should be combined using Bayes’ theorem Questionnaire prepared and given to the jury

27. R. v. D.J. Adams Retrial An example from the questionnaire: Bearing in mind that Ms Marley said that Mr Adams appeared to be in his early forties what is the probability that she would say that her assailant was in his early twenties if Mr Adams were indeed the assailant?

28. R. v. D.J. Adams Retrial Mr Adams was convicted. Mr Adams appealed

29. R v. Dennis John Adams Judgment: Second appeal ...the jury ...would have to ask themselves whether they were satisfied that only X white European men in the UK would have a DNA profile matching that of the rapist who left the crime stain It would be a matter for the jury, having heard the evidence, to give a value of X.

30. Dennis Adams judgment Match The probability of finding that profile if the crime stain had come from someone else is 1 in 200 million How many adult white males in the UK would have that profile? Approximately one tenth of a man

31. Well, that’s pretty straightforward, isn’t it? R v. Dennis John Adams It would be a matter for the jury, having heard the evidence, to give a value of X. X is approximately one tenth of a man They would then have to ask themselves whether they were satisfied that the defendant was one of those men.

32. This appears to be a deliberate instruction for juries to reason irrationally R v. Dennis John Adams ...consideration of the case along the lines indicated would, in our judgment, reflect a normal course for a properly instructed jury to adopt.... It is the sort of task which juries perform every day.

33. The present position If an expert is unable to carry out a statistical assessment of the weight of the evidence* then courts allow him to express an opinion of identity of source If an expert is able to carry out a statistical assessment of the weight of the evidence** then courts will not allow him to express an opinion of identity of source - the statistic is put to the jury * e.g. fingerprints, handwriting, toolmarks, footwear ** DNA profiling

34. The DNA experience suggests that if the weight of evidence can be measured by a numerical statistic.... It is our hope, as forensic scientists, that expert opinions will increasingly be based on databases and sound statistical methods ...then it is the statistic that should be put to the jury, rather than an expert opinion of identity of source

35. ...then it is the statistic that should be put to the jury, rather than an expert opinion of identity of source Most forensic scientists have no problem with this view... Nevertheless, it would seem desirable that we should explore with the judiciary and the legal profession ways of presenting statistical evidence... ...in a manner that is balanced, robust and logical

36. The most rational compromise would be to explain the meaning of the likelihood ratio And present illustrations based on a range of priors agreed by the court

37. That’s all folks!